1. Field of the Invention
The present invention relates to an active matrix type display device, having current driven light emitting elements provided for every one of pixels that are arranged in a matrix shape, for performing display by controlling current of the light emitting elements using drive TFTs that operate by receiving a data voltage at a gate.
2. Description of the Related Art
FIG. 1 shows the structure of a circuit for one pixel section (pixel circuit) of a basic active organic EL display device. An image data signal is stored in a storage capacitor C arranged across gate and source of a drive TFT 2 by setting a gate line (Gate), that extends in the horizontal direction, to a high level to turn a selection TFT 1 on, and in this state supplying an image data signal (also called data voltage) having a voltage corresponding to a display brightness, to a data line (Data) that extends in the vertical direction. In this manner, a drive TFT (a P-type TFT in this example) 2 having its source connected to PVdd supplies a drive current corresponding to the data signal to an organic EL element 3 that is connected to the drain of that TFT. As a result, the organic EL element 3 emits light in accordance with the data signal.
FIG. 2 shows one example of the structure of a display panel, and input signals. In FIG. 2, an image data signal, a horizontal sync signal (HD), a pixel clock and other drive signals are supplied to a source driver. Pixel data signals are sent to the source driver in synchronism with the pixel clock, held in an internal latch circuit once image data signals for a single horizontal line of pixels have been acquired, and subjected to D/A conversion simultaneously for supply to a data line (Data) of a corresponding row. Also, the horizontal sync signal (HD), other drive signals and a vertical sync signal (VD) are supplied to a gate driver 5. The gate driver 5 performs control to sequentially turn on gate lines (Gate) arranged horizontally along each line, so that image data signals are supplied to pixels of the corresponding lines. The pixel circuit of FIG. 1 is provided in each of the pixels 6 that are arranged in a matrix shape.
As a result of this type of structure, image data signals (data voltages) are sequentially written to each pixel in horizontal line units, and display is carried out at each pixel in accordance with the written image data signals, to perform image display as a panel.
Here the amount of light emission and current of the organic EL element 3 are in a substantially proportional relationship. Normally, a voltage (Vth) is supplied across the gate of the drive TFT 2 and PVdd such that a drain current approaching that for a black level of the pixel starts to flow. Also, the amplitude of the image signal is an amplitude so as to give a prescribed brightness close to a white level.
FIG. 3 shows a relationship for current “CV current” (corresponding to brightness) flowing in the organic EL element with respect to input signal voltage (voltage of the data line Data) of the drive TFT. It is possible to carry out appropriate gradation control for the organic EL element by determining the data signal so that Vb is supplied as the black level voltage and Vw is supplied as the white level voltage.